Personally I think 640 sheets is enough for anyone, however, rationality isn't rational at the moment.
Since The Thing is exponential there is a worst-case-scenario that everyone on the entire planet will have it by the middle of May, if not before.
So correctly assuming that you are the last person to get it, then you will need eight rolls of bathroom tissue. This will see you through to the calamity and through two weeks of self isolation to either recover or die.
If living in a house of two adults with two point two children, one dog and two cats, then you are going to need four of those nine roll packs to see The Thing through.
This has some contingency for problems of localised flooding in your bathroom, a stomach bug or children that are secretly selling your supplies.
Plus this allows for extra supplies in case vulnerable locked-down Baby Boomers run out in your neighbourhood and need the folding stuff.
Given that the hoarding mentality is growing at an exponential rate and that every tree on the planet could be cut down for bathroom tissue in a month's time, the people buying dozens of rolls are arguably behaving in a rational manner.